Transient or time domain electromagnetic (TEM) methods of geophysical exploration involve the inducement of eddy currents into the ground by an abrupt change in current flow in a transmitting antenna positioned in proximity with the ground. A receiving antenna detects the decay of the eddy currents as they diffuse outwardly and downwardly in the ground, away from the transmitting antenna. It is desirable to maximize the depth of exploration of TEM methods in order to maximize the effectiveness of TEM geophysical exploration. The depth of penetration of the usable TEM signal is the maximum depth reached by the eddy currents before their signal, as measured at the surface, becomes indistinguishable from noise. The longer in time the eddy current signal can be measured, the greater the depth of penetration. Typical depths of penetration obtained with conventional TEM methods are about 1 km.
Prior art TEM methods have concentrated on increasing the depth of penetration by using larger transmitter currents to increase the eddy current signal. However, significant increases in current, which are expensive to obtain, result in only relatively modest gains in the depth of penetration. This is because the depth of penetration is proportional to the signal-to-noise ratio raised to the 1/5 power. Thus, large increases in the signal-to-noise ratio are needed to obtain moderate increases in the depth of penetration.
A more economical and environmentally sound approach is to increase the depth of penetration by decreasing the noise. Prior art methods of decreasing noise involve stacking of successive data measurements; such methods are time consuming and, in particularly noisy areas, are ineffective in reducing noise to acceptable levels.
It is an object of the present invention to provide a method of reducing noise in TEM methods of geophysical exploration, thereby increasing the depth of exploration.